Chemical and Biological Effects of Radiation

Applications of Nuclear Chemistry: Radio-therapy; Diagnosis; Energy source; Geological dating; Biochemical studies; Food preservation (Read here about some "facts about food irradiation".)

Radioactivity: the ability to emit penetrating "rays" (radiations) that can expose wrapped photographic films, which is a process taking place in the atomic nuclous (by Ernest Rutherford)  Read a brief biography about this great physicist here, and about the discovery of alpha and beta radiation here.

Radioactive decay often resulting in the production (transmutation) of atoms of a different element (in order to gain stability of the radioactive isotope, or radionuclide).

Radioactive emissions (Table 10.1)
1) alpha particle (a ): He nucleus, thus is positively charged ()
The mass number decreases by 4, and the atomic number decrease by 2.

2) beta particle (b ): high energy electron formed by decomposition of a neutron into a proton and an electron, which can cause worse damage to our body than a radiation
The mass number is not changed, while the atomic number increase by 1.

3) positron: "positive electron" or "the antiparticle of the electron".
The mass number is not changed, while the atomic number decrease by 1.

4) gamma ray (g , often emitted along with a or b particles), X-ray (discovered by Wilhelm Roentgen by exposing metals to high electrical voltages), and cosmic ray (from outer space): They are all high-energy electromagnetic radiations.
No change in the mass number and atomic number.

Examples:

1) 

 

 

2) 

 

 

Other examples: Radium-230  undergoes beta emission (example 10.2)

3) 

4) 

 

 

Radioactive decay: loss of radioactivity by emitting radiation

Half-life (time), t1/2: the time required for half amount of a radionuclide to change to another element, which reflects the rate of decay of the radionuclide (Figure 10.1)
i.e., 100% amount (e.g., x g) t1/2-->50% left (100% ´ 1/2) or (x ´ 1/2) g left
    t1/2-->25% left (50% ´ 1/2 = 100% ´ 1/2 ´ 1/2 =100% ´ (1/2)2) or (x ´ (1/2)2) g left
    t1/2 >t1/2 >t1/2 > (total of n half-lives) 100% ´ (1/2)n left or (x ´ (1/2)n) g left

Thus, a beginning amount of 5.000 g after 5 half-lives has only 5.000 ´ (1/2)5 = 0.156 g left; or after 5 half-lives there is only 100% ´ (1/2)5 = 3.125% left, which is 5.000 ´ 0.03125 = 0.156 g.
Once you know the beginning amount of a radionuclide and its half-life time you can find the amount left after certain time period. (Note: The value "n" does not have to be an integer.)

<<Once again, here is another example that you need to know how to use your scientific calculator to enter the (1/2)n = 0.5n value.>>

Examples:
1) How much does 100 g I-131 remain after 32 days? 40 days? 100 days? (I-131, t1/2 = 8 days)
            Answers: 6.25 g, 3.13 g, and 100 ´ (0.5)100/8 = 100 ´ (0.5)12.5 = 0.0173 g

2) An amount of 10.0 mg I-131 is used for the treatment of thyroid disorder. How much amount is left after a month (30 days)?

3) Example 10.6, P. 265.
 
 

C-14 and archaeological dating (P. 267)
C-14 is generated at a constant rate in the upper atmosphere.
 

Thus, the ratio of C-14/C-12 is constant while organism is alive.
C-14 starts decaying after the organism dies.
 
    e.g., A piece of fossil of the saber-toothed tiger (cat) showed C-14/C-12 ratio to be only 1/4 that of fresh bones. When did the tiger die? (C-14, t1/2 = 5730 ± 40 years)
There is 25% C-14 left, which indicates 2 half-lives. Thus, the tiger died around 5730 ´ 2 = 11,460 years ago.  (For some more complicated calculations, take the challenge here!)

Effects of radiation
        H2O + radiation ® H2O+ + e
        H2O+ + H2O ® H3O+ + ·OH
The product ·OH (Hydroxyl radical) is a very active free radical, which can cause damages (cleavage of chemical bonds) on proteins, nucleic acids, and other biomolecules, and results in radiation sickness, including nausea and a drop in white blood cell count.
Fast growing or rapidly dividing cells are more accessible to radiation damages, including bone marrow, hair growing, intestinal lining, and cancers.

Detection of radioactivity: Geiger counter (pronounced as /gaiger/ in German), which is commonly used in the laboratories.

Measuring radioactivity (Units of radiation)
Curie (Ci): 3.7 ´ 1010 disintegration per second (dps), which is a measure of the number of atoms that decay per second, and is a measure rate of radioactive decay and is the number of disintegration of 1.00 g Ra per second.
SI unit: becquerel (Bq) = 1 dps
<<How many Cis is in 1 Bq?>>

roentgen (R): a measure of the ionization produced in air, which is also a measure of radiation capacity. 1 R = 2.1 ´ 109 ions in 1.0 cm3 of dry air at 0 ºC and 1.0 atm.

rad (Radiation Absorbed Dose): a measure of the energy absorbed in matter as a result of exposure to any form of radiation. 1 rad = quantity of ionizing radiation the delivers 0.100 J of energy to 1 kg of a substance.

rem (roentgen equivalent medical): a measure of the energy absorbed in matter as a result of exposure specific forms of radiation. (This unit is needed because different radiations afford different biological damages.) Thus, rem is equal to rad times a factor.

Radiation in medicine

    Diagnostic tools: X-ray (e.g., CAT scan, computer-assisted tomography) and PET scan (positron emission tomagraphy using a positron-emitting isotope with a short half-life <<Why?>>, such as O-13, N-13, C-11, and F-18)

    Radioactive tracers: such as Tc-99m which generates g -ray.

    Radiation therapy: Discussed above

Background radiation in the environment: cosmic rays, X-rays, and g -ray from K-40, Th-232, and U-238, and Rn-222 in the atmosphere, and so on.

Nuclear reactions: Occur when high-energy particles bombard the nucleus of an atom to produce a different nuclide/isotope.
Examples:

 

 

Nuclear fission: Atoms split into smaller "fragments" when bombarded
Example:

 

(The neutrons released then caused more U-235 to split, and generates a chain reaction. The energy generate by 1 kg U-235 is equivalent to 20 kilotons of dynamite, i.e. 20 ´ 106 times more).

Nuclear fission
Uncontrolled (virtually instantaneous): A-bomb
Controlled: nuclear power generator

Nuclear fusion